1. Field of the Invention
The present invention relates to a vehicle-mounted noise control apparatus for actively removing an effect of noises from outside a cabin of a motor vehicle.
2. Description of Related Art
As this kind of vehicle-mounted noise control apparatus, there has hitherto been known the following in International Publication No. WO 90/09655. Namely, in the apparatus, an audio signal which is outputted from an audio system such as a radio tuner, a cassette deck, or the like is amplified to a certain level. A noise control signal from an active noise control means is added to the amplified audio signal. The audio signal having the noise control signal added thereto is further amplified to thereby drive a loudspeaker. The active noise control means performs the following function as disclosed in Japanese Published Unexamined Patent Application No. 3084/1999. Namely, a noise from outside a motor vehicle is converted into a noise signal which is in the form of an electric signal. A signal which is obtained by filtering the noise signal through an adaptive filter is used as a control signal to thereby drive a control loudspeaker. The sound from the control loudspeaker is converted by an error detector into an error signal which is in the form of an electric signal. By using the adaptive filter, the error signal is arranged to become minimum all the time in response to the changes in the noise.
Therefore, if the control signal is outputted from a noise control means as the noise control signal and is further added to the audio signal as explained herein above, the noise can be aurally reduced by the noise control signal when music or the like is sounded from the loudspeaker.
In the vehicle-mounted noise control apparatus disclosed in the above International Publication No. WO 90/09655, there is a disclosure about how the connections are made among the constituting elements such as the audio system, the active noise control means, the amplifiers, mixing means, or the like. However, there is no disclosure about a concrete constitution of units each of which is made up of one or more constituting elements. Therefore, in case the active noise control apparatus as described in the above International Publication is carried into effect, there may be considered a constitution in which each of the constituting elements such as the audio system, the active noise control means, or the like is contained inside a single unit. On the other hand, various kinds of audio head units containing therein a radio tuner, a cassette deck, and other audio systems are put on the market for independent sale. It is therefore commonly practiced for a user to change those as originally mounted on a vehicle for other types which suit his own taste. If all of the constituting elements are contained inside a single unit as described above, there is a disadvantage in that they cannot be changed for a new audio head unit.
In view of the above disadvantage, the present invention has an object of providing a vehicle-mounted noise control apparatus in which the audio head unit can be replaced by the user.